The current direction of the oil and gas industry is to develop new offshore fields with multiple subsea tiebacks to host hubs, field centres or onshore facilities for final processing. This is often based on the requirement to make more effective use of existing infrastructure. One of the present key project stoppers for long distance tiebacks is incomplete and very expensive technology for avoiding problems with phase changes in the fluids, and possible deposition in pipelines. Cold flow, with slurry transport of solidified components, is an attractive solution, but it entails substantial challenges due to the phenomena associated with low-temperature fluid flow. Cold flow solutions may also be of high importance for many onshore field developments, as well as for long-distance transport of fully or partly processed liquid hydrocarbons.
Multiphase hydrocarbon wellstream transport exceeding the present-day transfer distances is of strategic importance for future deepwater field developments as well as being an enabler for economical exploitation of many marginal satellite fields and prospects at moderate water depths. Current technology for avoiding problems with e.g. wax or asphaltene deposition, or other solids, often entails adding significant amounts of inhibiting chemicals. This has a great impact on system economy, and is often also detrimental to local and/or global environmental aspects. Alternatively, pipelines for hydrocarbon transport may have to be thoroughly insulated or actively heated (both options are prohibitively expensive), regular scraping operations (pigging) may have to be conducted, or a large amount of fluid processing will have to take place close to the place of production, entailing e.g. complex offshore platform systems or large process facilities for onshore situations.
One challenging problem for cold flow is the presence of paraffinic wax and/or asphaltenes and/or other precipitating solids (i.e. “precipitors”) in many oil or condensate systems. When warm oil or condensate from e.g. a reservoir or from any other source of warm hydrocarbons is cooled down and/or the pressure is reduced, precipitors in the oil or condensate may become supersaturated and precipitate out as deposits on for example a pipe wall, or as solid particles/crystals suspended in the oil or condensate fluid. In some situations they may form a gel in the oil or condensate phase. Deposits of precipitors in pipelines can reduce production (by e.g. blocking conduits completely), reduce system regularity, and may increase costs through lost revenue and workovers by e.g. regular pigging of the pipeline.
When precipitors precipitate in an oil or condensate phase as small crystals or particles, they may be carried along with the hydrocarbon fluid without causing deposits or plugging. This is usually promoted by adding chemicals to the oil or condensate fluid before it is cooled to the crystallization temperature of the precipitors, or by mechanically dislodging deposits from surfaces after formation. From laboratory experiments, it is also well known that an increased supersaturation promotes crystallization of smaller precipitor particles inside the bulk of the oil or condensate phase.
U.S. Pat. No. 3,846,279 describes a method for producing wax slurries by using a fractionation tower and a water-filled reactor to produce wax particle slurries which can transport up to about 50% by weight of wax solids in the carrying oil. U.S. Pat. No. 3,910,299 makes use of essentially the same procedure, but with jacket circulation cooling instead of a water bath. A claimed wax fraction of up to 80% by weight is supposed to be transportable as a slurry after the process. Both these patents depend on a fractionation column being present upstream of the wax particle production equipment. U.S. Pat. Nos. 4,697,426 and 4,702,758 use shock cooling by choke expansion of gas or expansion turbines, respectively, to achieve quick formation of wax crystals, which are said to be transportable as a slurry thereafter. In U.S. Pat. No. 6,070,417 a process is described in which a fluid which may form solid deposits is circulated through a heat exchanger where large temperature gradients at the exchanger walls provide a tendency for solids formation to take place there. A runner designed to continuously circulate around in the heat exchanger, dislodges the solid depositions, ensuring that they are carried away into e.g. a pipeline at the outflow end of the heat exchanger. The same principle is explicitly proposed for wax deposition by Amin et al. (SPE paper 62947, ATCE Dallas, Tex., 1-4 Oct. 2000, 9 pp).
In Canadian patent no. 1,289,497, a method is given where a small amount of a cooled oil or condensate containing a large number of small wax particles or crystals suspended therein is added to a waxy oil or condensate at a temperature above the crystallization point of wax. Due to a higher melting point of formed wax the suspended wax particles will act as nuclei or centers of wax precipitation as the oil or condensate is thereafter cooled slowly below the crystallization point of wax. The cooled oil containing small wax particles or crystals may be obtained by withdrawing and cooling a small portion of the warm waxy oil or condensate before it is recycled into the warm waxy oil or condensate. To control wax formation by controlling the rate or degree of cooling of the main oil or condensate fluid is in Canadian patent no. 1,289,497 stated to be impractical or uneconomic.
British patent no. GB 2,358,640 described a method and system for transporting a flow of fluid hydrocarbons containing water, at elevated pressure. In the method, a flow of fluid hydrocarbons containing water at a temperature above the hydrate crystallization temperature is mixed with a cooled flow of fluid hydrocarbons containing gas hydrate particles. At the mixing point, the water from the warm fluid flow will moisten the dry hydrate particles from the cooled fluid flow. The temperature in the fluid flow after the mixing point is below the crystallization temperature of gas hydrates. The water-moistened dry hydrate particles in the fluid flow will—due to the supersaturation—quickly convert to dry hydrate without forming hydrate deposits on e.g. the pipe wall. The cooled fluid flow containing dry hydrate particles is obtained by recycling a sufficient part of the cooled mixed fluid flow. The amount of cooled fluid to be recycled is determined by the cooling required in order to obtain a mixing temperature close to the hydrate crystallization temperature.